asymcute: fix one byte out-of-bounds access in _len_get
asymcute: fix one byte out-of-bounds access in _len_get

As per Section 5.2.1 of the MQTT-SN specification, the MQTT-SN length
header is either 1- or 3-octet long. If it is 3-octet long then the
first octet is 0x01. The asymcute implementation currently only checks
that the incoming packet is at least 2-octet long before attempting to
parse it (MIN_PKT_LEN). However, if the first octet is 0x01 the packet
must be more than 3 octet long in order to be valid. Since asymcute
does not check this it reads one octet beyond the packet data for a
2-octet packet where the first octet has the value 0x01. This commit
fixes this issue by adding an additional sanity check to _len_get.

kaspar030/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,kYc0o/RIOT,miri64/RIOT,ant9000/RIOT,miri64/RIOT,kaspar030/RIOT,kYc0o/RIOT,jasonatran/RIOT,jasonatran/RIOT,miri64/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,ant9000/RIOT,kYc0o/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,ant9000/RIOT,ant9000/RIOT,jasonatran/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,kYc0o/RIOT,miri64/RIOT,RIOT-OS/RIOT,ant9000/RIOT,kaspar030/RIOT,jasonatran/RIOT,miri64/RIOT